Porous metal structures may be made by a variety of processes and used in a variety of situations. It has been found that porous metal structures are particularly useful for abradable seals, which are structures that readily wear at a rapid rate when contacted by a high velocity part, but which otherwise have integrity. They are especially used in turbomachinery. See for instance U.S. Pat. Nos. 4,049,428 Elbert et al and 3,111,396 Ball.
One of the favored methods for making porous metal structures is to form a compact of metal and fugitive material, and then cause the fugitive material to disappear, thus leaving a metal structure with less than full density. Conventional powder metallurgy techniques which involve making an admixture, pressing and sintering, have been used. See U.S. Pat. Nos. 3,864,124 Breton et al, 3,897,221 Salyer et al and the Ball and Elbert et al patents. See also U.S. Pat. No. 3,350,178 to Miller.
Because of the substantial shrinkage which sintering causes in a powder metal article, we have previously preferred using plasma arc spraying to make porous metal structures. During plasma spraying of a metal-polymer mixture, the metal particulates are bonded to one another. Thus, a subsequent sintering is either not needed or, if used, does not cause excessive shrinkage. Generally, a composite has been first made according to the teachings of Longo et al in U.S. Pat. No. 3,723,165 wherein a metal powder such as nichrome is sprayed along with a high temperature polyester powder, such as poly(paraoxybenzoyl), having a high melting point. The polyester softens but does not melt during spraying. While the particular step of making a porous structure by subsequently oxidizing the polymer is not mentioned in the Longo et al patent, we and others have done so in the making of experimental porous seal structures for gas turbine engines. Since the polyester used is a high temperature material, temperatures in the range of 540.degree. C. are needed to oxidize away the fugitive. However, while the abradable seals so made are effective for gas turbine engine use, the cost of the polyester resin particulate is high. Therefore, improvements have been sought, both to reduce costs and to improve the performance of abradable seals of porous metals by changing their physical and chemical characteristics.
Commonly owned U.S. patent application Ser. No. 406,404 filed Aug. 9, 1982 by Shiembob et al describes the use of a hard underlayer beneath a porous abradable material. Patent application Ser. No. 565,542 of Otfinoski et al filed on even date herewith, also commonly owned, describes and claims porous metal seal structures having certain advantageous combinations of density and oxide content, best produced by means of the present invention.